also consider "author" taxonomy query when testing whether we should add author opengraph metadata

I think we need @misterbisson's eyes on this one.

see https://github.com/GigaOM/gigaom/issues/4904
looks like setting `is_auithor` to TRUE in the `parse_query` callback isn't going to do it -- this tells `WP_Query` to look posts by that author _and_ have the coauthors-plus terms.

My latest idea is to install a filter that's called after the query is performed. [`the_posts`](https://github.com/GigaOM/legacy-pro/blob/master/application/wordpress/wp-includes/query.php#L3399) is the latest one to be called after `WP_Query` found all the posts I believe. We then set the `is_author` flag in the query back to `TRUE` if we detect the query contains a coauthors-plus taxonomy query. this works for the bsocial case in https://github.com/GigaOM/gigaom/issues/4904 at least.